Since the radar level gauging was developed as a commercial product in the 1970's and 1980's, frequency modulated continuous wave (FMCW) has been the dominating measuring principle for high accuracy applications. An FMCW measurement comprises transmitting into the tank a signal which is swept over a frequency range in the order of a few GHz. For example, the signal can be in the range 25-27 GHz, or 9.5-11 GHz. The transmitted signal is reflected by the surface of the contents in the tank (or by any other impedance transition) and an echo signal, which has been delayed a certain time, is returned to the gauge. The echo signal is mixed with the transmitted signal to generate a mixer signal, having a frequency equal to the frequency change of the transmitted signal that has taken place during the time delay. Due to the linear sweep, this difference frequency, also referred to as an intermediate frequency (IF), is proportional to the distance to the reflecting surface. The mixer signal is often referred to as an IF signal.
Although highly accurate, classic FMCW systems are relatively power hungry, making them less suitable for applications where power is limited. Examples of such applications include field devices powered by a two-wire interface, such as a 4-20 mA loop, and wireless devices powered by an internal power source (e.g. a battery or a solar cell).
In U.S. Ser. No. 12/981,995, by the same inventor, a novel and less power hungry measuring principle was introduced, involving emitting a series of pulses having constant carrier wave frequency, each pulse being long compared to the time of transit (e.g. a pulse duration in the order of 1 us to 100 ms, compared to time of transit in the order of tenths of a μs). The method is therefore referred to as a Multiple Frequency Pulsed Wave (MFPW).
The number of different carrier wave frequencies in a measurement cycle is insufficient to provide a continuous IF signal, or even an approximation of the IF frequency in the way done in so called “stepped” or “discrete” FMCW system where the steps are monotonous without power breaks to form a continuous signal. Instead, the small set of frequencies is chosen according to a specified frequency scheme, and a phase shift in the received pulse is determined for each frequency.
The process of determining the distance to the surface involves establishing a change of phase with emitted frequency (see FIG. 1). The line A represents an initial distance estimation, while line B represents an updated estimation. In theory, only two values (points x) are required to determine the rate of change (slope of line B), while in practice a larger number, e.g. a few hundred samples, may be required. Such a group of samples can be called measurement cycle and is a substitute for a FMCW-sweep. During a start up process (when no approximate distance is known) more samples are needed and this is also the case in more complicated cases (turbulence, disturbing echoes etc). As a start-up procedure samples like a rather conventional FMCW-sweep can be used.
In order to even further reduce power consumption it is desirable to only perform a complete distance measurement when required, i.e. when the surface has moved since the previous measurement.